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Create/Remove Exchange Database Availability Group

DAG Witness server is an ordinary server (not a member of dag or hosting any roles other than file share for dag)
It can be a witness for multiple DAGs
ADD DOMAIN\Exchange Trusted Subsystem group a member of .\Administrators group
The VBS script to achieve this is here (net localgroup is useless in this case)

Note: You might need to disable the witness firewall netsh advfirewall set allprofiles state off

Now you can remove your old entry from Active Directory forcefully, If you removed an Exchange server from your network without properly uninstalling it, either because you experimented with it on a test box or on a virtual machine, or simply because you didn’t know better, you may notice that the server continues to exist in the network’s Active Directory and shows up in the Exchange Management Console of your new exchange server.

Removing AD Configuration

The list of Exchange servers on the domain is stored in the Active Directory configuration. Following steps below to remove the obsolete server:

Log on to the Domain Controller.

From the Start menu select Run… and enter adsiedit.msc to launch the ADSI Edit tool

In the tool, right-click ADSI Edit and select Connect To…

In the Connection Settings dialog under Select a well known Naming Context select Configuration and click OK

Remove the computer from the members list of Exchange Trusted Subsystem.

If the computer was removed from the network altogether, you may also want to remove it from the Active Directory’s list of computers, as well as the DNS Server’s Forward Lookup Zones.

The DAG members in the primary datacenter must be forcibly evicted from the DAG's underlying cluster by running the following commands on each member:

net stop clussvc
cluster <DAGName> node <DAGMemberName> /forcecleanup

The DAG members in the second datacenter must now be restarted and then used to complete the eviction process from the second datacenter. Stop the Cluster service on each DAG member in the second datacenter by running the following command on each member:

net stop clussvc

On a DAG member in the second datacenter, force a quorum start of the Cluster service by running the following command:

net start clussvc /forcequorum

Open the Failover Cluster Management tool and connect to the DAG's underlying cluster. Expand the cluster, and then expand Nodes. Right-click each node in the primary datacenter, select More Actions, and then select Evict. When you're done evicting the DAG members in the primary datacenter, close the Failover Cluster Management tool.